U.S. planes pounded Islamic State positions in Syria for a second day on Wednesday, but the strikes did not halt the fighters' advance in a Kurdish area where fleeing refugees told of villages burnt and captives beheaded.
U.S. President Barack Obama, speaking at the United Nations, asked the world to join together to fight the militants and vowed to keep up military pressure against them.
"The only language understood by killers like this is the language of force, so the United States of America will work with a broad coalition to dismantle this network of death," Obama said in 40-minute speech to the U.N. General Assembly.
"Our strategy must work in tandem with Arab states, always in support of local people, in line with our legal obligations and as part of a plan that involves our aid, our diplomacy and, yes, our military," Cameron said at the U.N.
"We need to act and we need to act now," he said.
Syrian Kurds said Islamic State had responded to U.S. attacks by intensifying its assault near the Turkish border in northern Syria, where 140,000 civilians have fled in recent days in the fastest exodus of the three-year civil war.
Washington and its Arab allies killed scores of Islamic State fighters in the opening 24 hours of air strikes, the first direct U.S. foray into Syria two weeks after Obama pledged to hit the group on both sides of the Iraq-Syria border.
However, the intensifying advance on the northern town of Kobani showed the difficulty Washington faces in defeating Islamist fighters in Syria, where it lacks strong military allies on the ground."Those air strikes are not important. We need soldiers on the ground," said Hamed, a refugee who fled into Turkey from the Islamic State advance.
Mazlum Bergaden, a teacher from Kobani who crossed the border on Wednesday with his family, said two of his brothers had been taken captive by Islamic State fighters.
"The situation is very bad. After they kill people, they are burning the villages.... When they capture any village, they behead one person to make everyone else afraid," he said. "They are trying to eradicate our culture, purge our nation."
Fighting between Islamic State militants and Kurds could be seen from across the border in Turkey, where the sounds of sporadic artillery and gunfire echoed around the hills.
The United States said it was still assessing whether Mohsin al-Fadhli, a senior figure in the al Qaeda-linked group Khorasan, had been killed in a U.S. strike in Syria.
A U.S. official earlier said Fadhli, an associate of al Qaeda founder Osama bin Laden, was thought to have been killed in the first day of strikes on Syria. The Pentagon said any confirmation could take time.
Washington describes Khorasan as a separate group from Islamic State, made up of al Qaeda veterans planning attacks on the West from a base in Syria.
As Obama tried in meetings in New York to widen his coalition, Belgium said it was likely to contribute warplanes in the coming days, and the Netherlands said it would deploy six F-16s to support U.S.-led strikes.
The attacks have so far encountered no objection, and even signs of approval, from Assad's Syrian government. Syrian state TV led its news broadcast with Wednesday's air strikes on the border with Iraq, saying "the USA and its partners" had launched raids against "the terrorist organisation Islamic State."
U.S. officials say they informed both Assad and his main ally Iran in advance of their intention to strike but did not coordinate with them.
Even as Islamic State outposts elsewhere have been struck, the fighters have accelerated their campaign to capture Kobani, a Kurdish city on the border with Turkey. Nearly 140,000 Syrian Kurds have fled into Turkey since last week, the fastest exodus of the entire three-year civil war.
An Islamic State source, speaking to Reuters via online messaging, said the group had taken several villages to the west of Kobani. Footage posted on YouTube appeared to show Islamic State fighters using weapons including artillery as they battled Kurdish forces near Kobani. The Islamists were shown raising the group's black flag after tearing down a Kurdish one.
A Turkish official, speaking on condition of anonymity, said the advance had been rapid three days ago but was slowed by the U.S.-led air strikes.
But Ocalan Iso, deputy leader of Kurdish forces defending Kobani, said more militants and tanks had arrived in the area since the coalition began air strikes on the group.
"Kobani is in danger," he said.